Crystalline microporous molecular sieves, both natural and synthetic, such as zeolites, have been demonstrated to have catalytic properties for various types of hydrocarbon conversion processes. In addition, the crystalline microporous molecular sieves have been used as adsorbents and catalyst carriers for various types of hydrocarbon conversion processes, and other applications. These molecular sieves are ordered, porous, crystalline material having a definite crystalline structure as determined by x-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. The dimensions of these channels or pores are such as to allow adsorption of molecules with certain dimensions while rejecting those with larger dimensions. The interstitial spaces or channels formed by the crystalline network enable molecular sieves, such as crystalline alumino-silicates, to be used as molecular sieves in separation processes and catalysts and catalyst supports in a wide variety of hydrocarbon conversion processes.
Zeolites are comprised of a lattice of silica and optionally alumina combined with exchangeable cations such as alkali or alkaline earth metal ions. Although the term "zeolites" includes materials containing silica and optionally alumina, it is recognized that the silica and alumina portions may be replaced in whole or in part with other oxides. For example, germanium oxide, tin oxide, phosphorous oxide, and mixtures thereof can replace the silica portion. Boron oxide, iron oxide, titanium oxide, gallium oxide, indium oxide, and mixtures thereof can replace the alumina portion. Accordingly, the terms "zeolite", "zeolites" and "zeolite material", as used herein, shall mean not only molecular sieves containing silicon and, optionally, aluminum atoms in the crystalline lattice structure thereof, but also molecular sieves which contain suitable replacement atoms for such silicon and aluminum, such as silicoaluminophosphates (SAPO) and aluminophosphates (ALPO). The term "aluminosilicate zeolite", as used herein, shall mean zeolites consisting essentially of silicon and aluminum atoms in the crystalline lattice structure thereof.
The catalytic activity of many zeolites relies on their acidity. The substitution of silica with elements such as alumina with a lower valence state creates a positive charge deficiency, which can be compensated by a cation such as a hydrogen ion. The acidity of the zeolite can be on the surface of the zeolite and also within the channels of the zeolite. Within a pore of the zeolite, hydrocarbon conversion reactions such as paraffin isomerization, olefin skeletal or double bond isomerization, disproportionation, alkylation, and transalkylation of aromatics may be governed by constraints imposed by the channel size of the molecular sieve. Reactant selectivity occurs when a fraction of the feedstock is too large to enter the pores to react, while product selectivity occurs when some of the products cannot leave the channels. Product distributions can also be altered by transition state selectivity in which certain reactions can not occur because the reaction transition state is too large to form within the pores of the zeolite. Selectivity can also result from configuration constrains on diffusion where the dimensions of the molecule approach that of the pore system. Non-selective reactions on the surface of the molecular sieve, such as reactions on the surface acid sites of the zeolite, are usually not desirable as such reactions are not subject to the shape selective constraints imposed on those reactions occurring within the channels of the molecular sieve. Thus, resulting products produced by reaction with the surface acid sites of the zeolite are many times undesirable and can also deactivate the catalyst.
Many times it is desirable to carry out hydrocarbon conversion processes using large crystal zeolites. The term "large crystal" is used herein to mean the crystals have a diameter at least of about 2 microns. For example, large crystal zeolites have less specific outer crystal surface area which can reduce during hydrocarbon conversion the amount of reactions which take place on the surface of the zeolite. Furthermore, large crystal zeolites have longer diffusion path lengths which can be used to modify catalytic reactions. For instance, with respect to intermediate pore size zeolites such as a MFI type, increasing crystal size can change the selectivity of the catalyst when it is used in hydrocarbon conversion processes such as the disproportionation of toluene to paraxylene and the alkylation of aromatics. In the disproportionation of toluene to paraxylene, increasing the size of the zeolite crystal to lengthen the diffusion path increases paraxylene selectivity. The selectivity occurs because an increase in the diffusion constraints is imposed on the bulkier, slower diffusing o- and m-xylene isomers which reduces the production of these isomers and increases the yield of paraxylene isomer.